1. Technical Field
The present invention relates to an exposure head and an image forming apparatus which forms a latent image on a photoconductor by using lenses which image the light emitted from light emitting elements arranged in a predetermined pitch.
2. Related Art
As small light emitting elements are linearly arranged in a predetermined pitch and light emitted from each of the light emitting elements are imaged by lenses, it is possible to form one line of an imaged spot column on an image plane. Techniques for forming a latent image on a surface of a photoconductor using such principles have been developed. For example, in JP-A-2000-158705, it is possible to form a desired latent image by blinking the light emitting elements arranged linearly on an exposure head at an appropriate timing while rotating a cylindrically shaped photoconductor.
Also, in a case in which each of the light emitting elements has a dedicated imaging lens, the diameter of lens becomes small, so that it is not possible to increase the number of numerical aperture (NA number) of the lens. Accordingly, an exposure head which can secure a large NA number by the shared use of one imaging lens by a predetermined number of light emitting elements and thus considerably improve the resolution of latent image is proposed in, for example, JP-A-2008-036937. In this proposed exposure head, due to the following reasons, the groups of a predetermined number of light emitting elements (hereinafter, referred to “light emitting element array”) are obliquely arranged differently from each other. First, the end portion of the lens is not provided with a light emitting element array since it has a low imaging capability. Thus, first, imaging lenses are linearly arranged and a predetermined number of light emitting elements (light emitting element array) are arranged only in the vicinity of a center portion of each lens. In this state, since the boundary line portion between the lenses is not provided with the light emitting element array, a similar imaging lens column is arranged in the immediate vicinity of the imaging lens column with a slight deviation, and the boundary line portion between the lenses is filled by light emitting element array of a newly arranged imaging lens column. Focusing on the light emitting element array, a plural of light emitting element arrays are arranged in a zigzag pattern. Of course, if the addition of only one line of a new imaging lens column is not sufficient to accomplish the filling in, more imaging lens columns may be added. In this case, the plural number of light emitting element arrays are repeatedly arranged obliquely with a positional deviation with each other.
Further, in the exposure head in which the light emitting element arrays are arranged to be different from each other, if the exposure head is obliquely assembled in a plane parallel to the surface of the photoconductor (in a state that it is rotated about an axis having a direction towards the photoconductor), it is possible to see the emitting element arrays arranged differently from each other in the oblique direction, and thus the sections in which a space are generated between the light emitting element arrays widens and/or narrows. As a result, the section in which groups of imaging spots formed by each light emitting element array are separated and its near section are generated in a latent image of the photoconductor. Alternatively, in a case of forming a long imaging lens column by connecting a plurality of relatively short imaging lens columns rather than forming an imaging lens column integrally, a deviation in a lens pitch occurs at a connected portion, which causes the occurrence of the sections in which the distance between the spot groups has widened or narrowed, and thus it is difficult to form a good latent image.
In consideration of the above description, another technique, for example, in JP-2008-173889 is known, in which an end of a light emitting element array is slightly extended and a light emitting element overlapped with the emitting element of another light emitting element array (an overlapped element) is provided. In a case in which the distance between the spot groups is widened and thus a gap appeared, the gap is filled by forming a spot by means of the overlapped element, whereas in a case in which the distance between the spot groups is narrowed, a spot in that portion is thinned out, whereby degradation of image quality of a latent image is avoided.
However, there is a problem that it is difficult to obtain a sufficiently good latent image only by forming a spot by the overlapped element provided at an end of the light emitting element array or thinning out the spot. The reason is that, as is apparent from the above-described mechanism, even when the distance between the spot groups is widened, the distance can have various values. For example, in a case of forming a spot of the overlapped element since a space between the spot groups is slightly widened, the pitch of the spot at that portion is conversely narrowed. Further, there occurs a case in which providing one spot of the overlapped element is not sufficient, but providing two spots of the overlapped element is excessive. Similarly, in a case of thinning out one spot since a distance between the spot groups has narrowed, the pitch of the spot at that portion is conversely widened. Of course, there occurs a case in which thinning out one spot is not sufficient, but thinning out two spots is excessive.